I was in a similiar position not long ago. I have a 2007 Vulcan 2000 Classic LT and couldn't stand the stock position of the license plate.
Didn't find anything online that I liked and wanted a side mount. Having the added benefit of having bags on this bike, I make a couple of "'L" brackest to go on the inside of the bag from the bag attachment points to the bottom of the bag and made another "L" bracket that would go longitudinally under the bag and provide a place to mount the license plate. Obviously, this means I had to drill 2 holes in the bottom of the bag to bolt everything together, but since I sealed everything up with silicone, the structure and integrity of the bag was not comprimised.
Another reason I went this route and stopped searching for a swing arm/axle mount mount type was I didn't want the plate constantly impacting the saddel bag above it.
This may not work for you since you didn't specify if you had saddle bags or not, but I thought I would throw this out there as one mans option.
Hope this help and if you want pics, let me know and I will post a few. (If I can figure out that picture taking thingy...)
Not all that new here, just haven't felt the need to post.